Woofer is the term used for an active loudspeaker driver or transducer designed to produce low frequency “bass” sounds, typically for frequencies between approximately 20 Hz and 250 Hz. Within the lower part of this range, a type of woofer termed a subwoofer is designed to handle the lowest two or three octaves (e.g., between about 20 Hz-120 Hz). It is not unusual for some subwoofer systems to extend to frequencies an octave or more below 20 Hz.
The woofer transducer includes a diaphragm or cone with a flexible surround or suspension driven by a voice coil attached thereto, where the voice coil is surrounded by a motor assembly which generates a magnetic field. When current flows through the voice coil, the coil moves and causes motion of the diaphragm, creating sound waves as the diaphragm moves inward and outward. In order to have reliable sound production, the motion of the diaphragm must be controlled so that the electrical signal to the woofer's voice coil is accurately reproduced by the sound waves produced by the diaphragm's motion.
The transducer is typically mounted within an enclosure or box which couples the diaphragm motion to the air inside the enclosure. In a sealed enclosure, the transducer interacts with a trapped volume of air in the enclosure, such that as the woofer diaphragm moves outward it decreases the air pressure inside the enclosure, and as the woofer diaphragm moves inward it increases the air pressure inside the enclosure. In ideal conditions, this air pressure acting on the woofer's diaphragm from inside the enclosure will be the same as the air pressure acting on the woofer's diaphragm from outside the enclosure, such that both inward and outward diaphragm motion has a symmetrical characteristic. Maintaining a stable, symmetrical and linear pressure within the enclosure is important in order to reliably reproduce sounds with low distortion.